This study is designed to measure the single dose and steady state half-lives of mephenytoin and its metabolite, nirvanol, and to determine the rate of biotransformation of mephenytoin to nirvanol. Mephenytoin is a widely used antiepileptic drug whose side effects are thought to be more severe than that of the more commonly used hydantoin, phenytoin. Patients who are not on mephenytoin, whose seizures might respond to a hydantoin, but who are refractory to phenytoin are included in the study. Patients are admitted to the Clinical Center and enter a week long base-line period of observation on their present regimen, including special studies such as intensive monitoring with videotape analysis and long-term EEG recording and frequent levels of current antiepileptic medication. Following the base-line week, mephenytoin is studied after both chronic and single dose administration. Patients whose seizures respond to mephenytoin are encouraged to continue this medication.